A swash plate type piston pump/motor comprises, as shown in FIG. 3, for example, a cylinder block 3 mounted in a housing 1 so as to rotate freely together with a rotating shaft 2, and a plurality of pistons 5 slidably inserted in a plurality of cylinder holes 4 formed in the cylinder block 3 so as to define a plurality of cylinder chambers 6, the arrangement being made such that each of these pistons 5 is reciprocated in each of the cylinder holes 4 by slidably moving each of the pistons 5 along the front surface 7a of a swash plate 7 so as to increase and decrease the volume of each of the cylinder chambers 6, and the displacement of piston pump/motor is determined by the stroke of reciprocation of the pistons 5, and in order to increase the displacement, the angle of tilting of the swash plate 7 is varied so as to increase the stroke of reciprocation of each piston 5.
As examples of the apparatus for varying the angle of tilting of the swash plate 7, there are heretofore known an apparatus wherein the semi-circular surface 7b (in sectional view) of a swash plate 7 is swingable along a semi-circular recess 8a formed in a cradle 8, and the swash plate 7 is swung by adjusting pistons 9 along the cradle 8, as shown in FIG. 3, and an apparatus as shown in FIG. 4 wherein a swash plate 7 has a first contact surface 10 and a second contact surface 11 formed on the rear surface 7c thereof, and a ball 12 is provided over a swash plate receiving surface 1a of a housing 1 and the boundary portion between the first and second contact surfaces, the arrangement being made such that when the swash plate 7 is swung by an adjusting piston 13 around the ball 12, the first and second contact surfaces 10 and 11 are brought into contact with the swash plate receiving surface 1a to vary the angle of tilting of the swash plate 7.
The former apparatus is complicated in construction and has slidably movable parts which offer a high resistance to sliding movement, and so its operation is liable to become unreliable, whilst the latter apparatus is simple in construction, but the angle of tilting angle of the swash plate cannot be increased to a large extent, and if the angle of tilting of swash plate is increased appreciably, the overall size of the piston/pump motor assembly becomes large.
Stating in brief, since the swash plate receiving surface 1a intersects the axis 2a of the rotating shaft 2 at right angles, when the first contact surface 10 or the second contact surface 11 is brought into contact with the swash plate receiving surface 1a, either one of these contact surfaces intersects the shaft 2 at right angles. Thus, if the angle of tilting .theta. of the swash plate is increased, then the center 12a of the ball 12 will move away from the front surface 7a of the swash plate 7, i.e., the surface on which the pistons 5 are allowed to slidably move, so that the amount of eccentiricity L.sub.2 from the axis 2a of the rotating shaft 2 to the center 12a of the ball 12 will increase, and the point of action of the resultant force Fp of forces exerted by the pistons 5 will deviate a little due to rotation of the swash plate 7. Therefore, the position of the center 12a of the ball 12 must be decided taking into account the amount of deviation L.sub. 1.
If the distance between the center 12a of the ball 12 and the axis 2a of the rotating shaft 2 is increased, then it is necessary to increase the swinging force of the swash plate 7 caused by the adjusting piston 13. To stabilize the swash plate 7, it is necessary to increase the distance from the center 12a of the ball 12 to the end 11a of the second contact surface 11 to thereby support the swash plate 7 so as to prevent it from turning over due to the thrust exerted by the adjusting piston 13. As a result, the length L.sub.3 from the center 2a of the rotating shaft 2 to the end of the swash plate 7 becomes longer so that the size of the housing 1 becomes bigger correspondingly, thus increasing the overall size of the piston pump/motor.